High pressure tree cap

ABSTRACT

A wellhead cap assembly (10) includes a high security barrier (16) for installation onto a wellhead (12) containing any leakage safely and for permitting the installation of a subsea xmas tree onto the wellhead (12) whilst, simultaneously, providing means for containing full pressure control during rectification of the leak. The barrier (16) is provided by an internal tree cap which allows pressure between the top of the tubing hanger (34) and the underside of the tree cap (16) to be monitored by pressure monitoring means (20) via a port (29) in the assembly (10) and, in addition, in the event the presence of well bore fluids is detected indicating that leakage has occurred, the cap assembly allows intervention in order to rectify the leakage.

The present invention relates to well safety and, in particular, tosafety of perforated and suspended wells in which are installed tubinghangers fitted with wireline plugs, but which do not have a xmas treefitted.

At the end of the drilling phase of a production well, a tubing hangermay be run into the well, while the BOP stack and marine riser are stillconnected. The tubing hanger is landed, locked and then pressure tested.Downhole work may then occur to clean the well up and a brief well testmay then be conducted. At the end of this phase the well is made safeprior to the disconnection and retrieval of the BOP stack and marineriser, to await the installation of a subsea xmas tree on some occasionin the future. The well is made safe by the installation of mechanicalplugs with elastomeric seals deployed into the well using wirelinetechniques.

Within the production bore a primary barrier is established by theinstallation of a deep set plug in the downhole completion, at or belowthe packer assembly. A secondary barrier is established by theinstallation of a second similar plug within a suitable profile in thetubing hanger production bore. On the annulus side, the primary barrieris the downhole packer assembly and a secondary barrier is establishedby the installation of a similar wireline set plug in a separate profileof the annulus bore within the tubing hanger. Thus, the statutoryrequirement to provide two barriers between the reservoir and theenvironment are satisfied.

However, to date, the authority responsible for safety in the Britishsector of the North Sea, for the time being, the Health & SafetyExecutive (HSE), requires there to be a drilling facility on hand in thelocality for the duration that a live well is suspended in this mannerwith a dual bore tubing hanger. The reason for this is that wireline setplugs have a poor record of reliability with the potential for suchplugs to leak. Maintaining the presence of a drilling facility in thelocality of such a suspended well enables the rig to stop its currentactivity and return to the suspended well with leaking plugs, re-run aBOP stack and perform the work necessary to stop the leakage. It will beappreciated that for field developments where such a drilling facilityis on hand during the development of the field, no problem exists, therig is in the area drilling other wells while previously drilled wellsare suspended in this manner.

However, for small field developments, this requirement limits theopportunities to suspend wells in such a manner. It may not be possibleto ensure the availability of a rig in close proximity to the suspendedwell for the period in which the well would be suspended, and it wouldnot be cost effective to maintain a dedicated rig available in thelocale until such time as subsea xmas trees are installed onto thesuspended wells.

An object of the present invention is to obviate the requirement for arig to be on hand in locales where there are suspended drilled wells.

A further object of the invention is to provide a barrier which can becoupled to the well to prevent leakage such that a drilling vessel isnot required and which barrier can be easily removed to accommodate theinstallation of a subsea xmas tree.

This is achieved by providing an additional high security barrierinstalled onto the wellhead for containing any leakage safely and whichpermits the installation of a subsea xmas tree onto the wellhead whilst,simultaneously, providing means for containing full pressure controlduring rectification of the leak. The barrier includes means formonitoring the pressure between the upper surface of the tubing hangerand the underside of the barrier and, in addition, in the event thepresence of well bore fluids is detected indicating that leakage hasoccurred, the barrier includes means to allow intervention in order torectify the leakage.

According to a first aspect of the present invention there is provided awellhead security system comprising,

a wellhead cap assembly for fitting onto a wellhead, the wellhead havinga tubing hanger assembly suspended therein, the tubing hanger assemblyhaving plugs disposed in the production bore and the annulus bore,

the wellhead cap assembly having a removable internal cap for providingan additional barrier to the tubing hanger plugs, a chamber definedbetween the internal cap and the top of the tubing hanger, and pressuremonitoring means coupled to the tree cap assembly for monitoringpressure within said wellhead cap assembly indicative of the leakage ofwell fluid past the tubing hanger into said chamber.

Preferably, the wellhead cap assembly is dimensioned and proportioned toreceive a blow-out preventer (BOP) stack and an internal tree capretrieving tool moveable through the bore of said BOP stack forretrieving the internal tree cap from the cap assembly for replacementor restitution of the tubing hanger plugs, or other components which arefound to have given rise to the leak.

Preferably also, the pressure monitoring means is integral with the treecap assembly. Alternatively, the pressure monitoring means is separatefrom the tree cap assembly. Conveniently, the pressure monitoring meansalso includes vent means which is actuatable to vent excess fluidpressure from said chamber.

Preferably also, the pressure monitoring means includes a pressuremonitoring bore which passes through the wall of the wellhead capassembly by which fluid pressure can be monitored in said chamber.

According to another aspect of the present invention, there is provideda method of providing an additional safety barrier on a wellhead inwhich a tubing hanger is suspended with plugs therein, said methodcomprising the steps of,

disposing a tree cap assembly on top of the wellhead and creating anadditional barrier above the tubing hanger assembly,

defining a chamber between the additional barrier and the top of thetubing hanger, and providing means for monitoring fluid pressure withinsaid chamber indicative of leakage of wellhead fluid into said chamberpast said tubing hanger set plugs.

Preferably also, said method includes the step of venting excesspressure from said chamber when said monitored pressure exceeds apredetermined value.

Preferably also, the method includes coupling a BOP stack to thewellhead cap assembly and providing tree cap retrieval means passingthrough the BOP stack for removing the additional barrier to repair orreplace said tubing hanger set plugs in the event that leakage of wellfluid is present.

These and other aspects of the present invention will become apparentfrom the following description when taken in combination with theaccompanying drawings in which:

FIG. 1 is a diagrammatic elevational representation of a high pressuretree cap assembly in accordance with an embodiment of the presentinvention;

FIG. 2 is a view similar to FIG. 1 except that the high pressure capassembly is shown installed on a well suspended with a dual bore tubinghanger, and

FIG. 3 shows a BOP stack installed on the high pressure cap assemblywith a tool deployed for the retrieval of the internal tree cap.

Reference is first made to FIG. 1 of the drawings which depicts a highpressure tree cap assembly generally indicated by reference numeral 10which incorporates a completion spool assembly comprising a wellheadconnector 12 with a 183/4" mandrel 14 attached to it at its uppersurface, an internal tree cap 16 and a pressure gauge and vent facility,generally indicated by reference numeral 20. The upper area of thecompletion spool bore 22 accommodates the internal tree cap 16. A lowerseal is established by a full bore metal seal 24 (AX/VX gasket) which isdisposed between the mandrel 14 and the wellhead. Seals 17 located onthe external surface of the internal tree cap 16 establish the upperseal between the tree cap 16 and the mandrel bore 22. By these means anyleakage from tubing hanger plugs (not shown) is contained within thehigh pressure cap assembly 10. The high pressure gauge monitoring means20 includes suitable isolating valves 26 and vent valves 28 which areprovided to monitor the internal pressure in bore 22 through port 29 inthe mandrel 14 within the high pressure cap 10, and the external topsurface 30 of the mandrel provides a suitable proprietary profile (VetcoH4, Drillquip profile or clamp hub) onto which a BOP stack (not shown)may be attached for intervention purposes.

Reference is now made to FIG. 2 of the drawings which depicts the highpressure cap assembly 10 located in position on a wellhead 32 in which atubing hanger 34 is located, having a main production bore 36 and anannulus bore 38 into which tubing hanger wireline plugs 40 and 42 aredisposed respectively. The wellhead 32 typically an 181/4" wellheadwhich mates with the internal diameter of the wellhead connector 12. Itwill be seen that, in use, the full bore metal seal 24 is located aroundthe tubing hanger. The tree cap 16 is located above the top of thetubing hanger 34. In the event that there is fluid or gas leakage, pastor through the tubing hanger 34, so that there is a pressure increase inthe bore 22 between the upper surface of the tubing hanger and theunderside of the tree cap 16, the pressure therein can be monitored viaport 29 using the pressure gauge and vent facility 20 (FIG. 1). If thereis excess pressure, the vent valve 28 can be actuated to vent theinternal pressure from the high pressure cap assembly 10.

Reference is now made to FIG. 3 of the drawings which depicts a BOPstack, generally indicated by reference numeral 44, attached to the highpressure cap assembly 10 with a tool 46 deployed for the retrieval ofthe internal tree cap 16. In addition to the parts shown and describedin FIGS. 1 and 2, it will be seen that the BOP connector 48 of BOP stackfits over the mandrel 14 and the high pressure cap assembly 10. It willalso be seen that the BOP stack 44 is coupled to a marine riser 50 via aflex joint 52. Within the bore 54 of the BOP stack and marine riser isthe internal tree cap deployment tool 46 which allows the tree cap 16 tobe retrieved to allow replacement or rectification of the tubing hangerplugs 40,42. Once the internal tree cap 16 has been retrieved, a toolfor intervening on the tubing hanger (not shown) may then be run on dualbore riser (not shown) and any leakage at the plugs 40,42 may berectified with the aid of wireline techniques as is currently commonpractice.

Various modifications may be made to the embodiment hereinbeforedescribed without departing from the scope of the invention. Forexample, the full bore metal seal 24 may be replaced by any othersuitable seal; it does not have to be metal. In addition, it will beunderstood that the size of the cap assembly can be varied to suitvarious wellhead sizes, although 183/4" H4 (or CIW) connectors are mostcommon. Similarly, the mandrel profile can be varied to suit varioussizes of BOP stacks, although the 183/4" mandrel H4 (or CIW) profile isthe most common. The pressure gauge and vent facility 20 can be coupledto the cap assembly 10 or can be separate and coupled to the guideframe. If separate, it can be coupled to the tree cap assembly 10 wheninstalled as shown in FIG. 1. It will be understood that the tree capassembly 10 has applications with conventional subsea test tree andtubing hanger running tools and not only the 5"×2" completion subseatest tree, although it should be understood that the use of the 5"×2"completion subsea test tree offers a number of advantages which aredocumented in applicant's co-pending Application No. 9509547.7. Thetubing hanger may also be of a design different from the dual bore type,but in all cases there will be a production bore which may be isolatedby means of wireline plugs. The bore referred to as production may alsobe designated for the injection of either water or gas into a reservoir.

The principal advantage of the present invention is that a tertiarybarrier is provided to contain any leakage through the first andsecondary barriers. In addition, the cap assembly allows the monitoringof pressure between the upper surface of the tubing hanger and theunderside of the extra barrier. A single bore or multiple bores may passthrough the wall of the wellhead assembly for monitoring pressure, orthis can be provided by a single bore to monitor pressure and a separatevent bore which has a valve activated by the pressure measured to ventexcess pressure from inside the assembly. Furthermore, in the event thatwellbore fluids are detected within this space, indicating a leakage hasoccurred, intervention can be required to rectify the leakage. The capassembly simultaneously provides facility for attachment of a suitablemeans or maintaining full pressure control during rectification of theleak by allowing a BOP stack to be coupled to the top of the capassembly. In addition, once the well is secured against leaks, thebarrier may be removed immediately prior to the removal of the BOP stackand the installation of a subsea xmas tree, thereby providing securityagainst leaks during the period of well suspension. Accordingly, thisprovides the significant advantage that the presence of the drillingfacility in the locality while the well is suspended is not required.

What is claimed is:
 1. A wellhead security system comprising,a wellheadcap assembly for fitting onto a wellhead, the wellhead having a tubinghanger assembly suspended therein, the tubing hanger assembly havingplugs disposed in a production bore and an annulus bore, the wellheadcap assembly having a removable internal cap for providing an additionalbarrier to the tubing hanger plugs, a chamber defined between theinternal cap and a top of the tubing hanger assembly, and pressuremonitoring means coupled to the wellhead cap assembly for monitoringpressure within said wellhead cap assembly indicative of leakage of wellfluid past the tubing hanger assembly into said chamber.
 2. A system asclaimed in claim 1 wherein the wellhead cap assembly is dimensioned andproportioned to receive a blow-out preventer (BOP) stack and an internalcap retrieving tool moveable through a bore of said BOP stack forretrieving the internal cap from the cap assembly for replacement orrestitution of the tubing hanger plugs.
 3. A system as claimed in claim1 wherein the pressure monitoring means is integral with the capassembly.
 4. A system as claimed in claim 1 wherein the pressuremonitoring means is separate from the cap assembly.
 5. A system asclaimed in claim 1 wherein the pressure monitoring means includes ventmeans which is actuatable to vent excess fluid pressure from saidchamber.
 6. A system as claimed in claim 1 wherein the pressuremonitoring means includes a pressure monitoring bore which passesthrough a wall of the wellhead cap assembly by which fluid pressure canbe monitored in said chamber.
 7. A method of providing an additionalsafety barrier on a wellhead in which a tubing hanger is suspended withplugs therein, said method comprising the steps of,disposing a tree capassembly on top of the wellhead and creating an additional barrier abovethe tubing hanger assembly, defining a chamber between the additionalbarrier and a top of the tubing hanger, and providing means formonitoring fluid pressure within said chamber indicative of leakage ofwellhead fluid into said chamber past said tubing hanger plugs.
 8. Amethod as claimed in claim 7 wherein said method includes the furtherstep of venting excess pressure from said chamber when said monitoredpressure exceeds a predetermined value.
 9. A method as claimed in claim7 including the further steps of coupling a BOP stack to the tree capassembly and providing tree cap retrieval means for passing through theBOP stack for removing the additional barrier to repair or replace saidtubing hanger plugs in the event that leakage of well fluid is present.